Water is one of the key molecules in the physical and chemical evolution of star- and planet-forming regions. We here report the first spatially resolved observation of thermal emission of (an isotopologue of) water with the Plateau de Bure Interferometer toward the deeply embedded Class 0 protostar NGC 1333-IRAS4B. The observations of the H¹⁸ ₂O 3 _1,3-2_2,0 transition at 203.4 GHz resolve the emission of water toward this source with an extent of about 02 corresponding to the inner 25 AU (radius). The H¹⁸ ₂O emission reveals a tentative velocity gradient perpendicular to the extent of the protostellar outflow/jet probed by observations of CO rotational transitions and water masers. The line is narrow, ≈ 1kms^-1 (FWHM), significantly less than what would be expected for emission from an infalling envelope or accretion shock, but consistent with emission from a disk seen at a low inclination angle. The water column density inferred from these data suggests that the water emitting gas is a thin warm layer containing about 25 M _Earth of material, 0.03% of the total disk mass traced by continuum observations.